1. Field of the Invention
The present invention relates to a thin film semiconductor device (e.g., a field effect transistor or "FET") for use as a switching element to be provided on an insulative substrate (e.g., a glass substrate of an active matrix type liquid crystal display device) corresponding to each pixel of a display device, or as an element of a liquid crystal driving circuit for driving the respective pixels; and a method for producing the same. In particular, the present invention relates to a thin film semiconductor device incorporating an active layer composed essentially of polycrystalline silicon or crystalline silicon formed by laser annealing; and a method for producing the same.
2. Description of the Related Art
Conventionally, thin film transistors (hereinafter referred to as "TFTs") are known as one example of thin film semiconductor devices incorporating an active layer composed essentially of polycrystalline silicon. A TFT is typically produced by irradiating an amorphous silicon layer or a polycrystalline silicon layer with a laser beam so as to thermally melt the amorphous or polycrystalline silicon layer, and subsequently cooling the melted layer so as to allow it to recrystallize.
However, the conventional method often results in a crystal layer including non-uniform crystal grain sizes, and/or a large number of grain boundaries may occur in the resultant crystal layer. A TFT incorporating such a crystal layer is likely to have insufficient transistor characteristics. A liquid crystal display device incorporating such transistors would generally have insufficient display quality.
On the other hand, the formation of grain boundaries at least partially depends on the conditions for the recrystallization of the melted silicon. In particular, grain boundaries are known to most likely occur in regions having a non-uniform temperature distribution (for example, different melt temperatures are observed in the center of a laser beam spot and the peripheral portions thereof). Therefore, as one method for overcoming the above problem, it has been desired to perform recrystallization while maintaining the temperature distribution as uniform as possible in the regions which are require to be recrystallized. For example, a method has been proposed for controlling the temperature of the silicon layer in both a step of melting the silicon by laser beam irradiation and a step of cooling the silicon layer. Both of the following two conditions are supposed to be satisfied:
1) the thermal distribution of the still-melted silicon obtained by a melt process with laser beam irradiation must become uniform; and PA1 2) the silicon temperature must be gradually lowered when recrystallizing the silicon in a melt state, thereby obtaining relatively large grain diameters.
However, it has been difficult to simultaneously satisfy the above two conditions for the following reasons. In terms of making the thermal distribution of the melted silicon uniform, a substrate having high thermal conductivity is generally preferable (so as to achieve sufficient thermal draining toward layers underlying the silicon layer). On the other hand, in terms of controlling the cooling speed of the melted silicon, a substrate having high thermal conductivity is generally unpreferable because the radiation from the silicon should be minimized.
A few examples of conventional thin film semiconductor devices will be described below.
Japanese Laid-Open Patent Publication No. 6-34997 discloses a method in which a light-shielding layer formed of a metal having a high melt point is formed under a silicon layer, with an interlayer insulation layer interposed therebetween. According to this method, thermal draining occurs to a large extent in the vicinity of the light-shielding layer at the time of laser beam irradiation, so that a relatively uniform thermal distribution can be obtained in the vicinity of the beam spot. However, this method is disadvantageous due to the large radiation from the silicon layer, which allows the melted silicon layer to cool down relatively quickly, thereby resulting in small grain diameters and insufficient uniformity of crystal grain size.
Japanese Laid-Open Patent Publication No. 6-291034 discloses a method in which a silicon oxide layer and a heating layer of Ge or Mo are in turn formed, the heating layers being irradiated with a second laser beam having a different wavelength than that of the laser beam for annealing the silicon layer, thereby providing an effective thermal processing for the silicon layer. According to this method, the heating layers are additionally formed, which are subjected to further laser irradiation, so that this method may be effective in terms of controlling the thermal distribution of the silicon layer in a melt state and the cooling rate of the silicon layer from the melt state. However, this method may be disadvantageous due to the complex conditions required for the laser irradiations with different wavelengths.
Japanese Laid-Open Patent Publication No. 6-29321 discloses a method in which a cavity is formed under a silicon layer. According to this method, the cavity functions to control the radiation from the silicon layer (which has absorbed laser energy) into the insulative substrate, thereby effectively increasing the resultant grain diameters. However, this method results in a non-uniform thermal distribution of the melted silicon layer due to the decreased thermal conduction from the silicon layer.
Japanese Laid-Open Patent Publication No. 6-132306 discloses a method in which a thin silicon oxide film is formed under a silicon layer, a thin silicon nitride film being further formed under the thin silicon oxide film. According to this method, the relatively low thermal conductivity of the silicon oxide layer prevents the temperature of the melted silicon (which was melted by laser irradiation) from rapidly decreasing, so that the grain diameters may be somewhat increased. However, as in the case of Japanese Laid-Open Patent Publication No. 6-29321, this method results in a non-uniform thermal distribution of the melted silicon layer due to decreased thermal conduction from the silicon layer.